1. Field of the Invention
The present invention relates generally to digital communications over an Asynchronous Transfer Mode (ATM) communications network.
2. Related Art
A communications network serves to transport information among a number of locations. The information is usually presented to the network in the form of time-domain electrical signals and can represent any combination of voice, video, or computer data. A typical communications network consists of various physical sites called "nodes", interconnected by conduits called "links". Each link carries information from one site to another site. Individual sites contain data terminating equipment (DTE) for combining, separating, and transforming data.
T1 (also known as DS1) is one type of digital communications link. T1 is a synchronous link capable of carrying 24 DS0 channels which are time domain multiplexed (TDM) and transmitted over a single physical line. A DS0 channel is a 64 kilobites per second (64 Kbps) channel, which is the world wide standard for digitizing voice conversation. This occurs because an analog voice signal can be adequately represented by a digital data stream if sampled at a rate of 8000 samples per second. If each voice sample is digitized using 8 bits, this results in a digital data stream of 64 Kbps.
A T1 link transmits one T1 frame 8000 times per second (or one frame every 125 .mu.s). Each T1 frame contains a T1 payload with 24 DS0 timeslots, one for each DS0 channel with 8 bits in each timeslot. Each T1 frame also has T1 frame bit that identifies the start of the T1 frame, so that a T1 frame has a total size of 193 bits. This results in a data stream of 1.544 Mbps (8000 frames/sec.multidot.193 bits/frame).
A T1 superframe is a group of 12 T1 frames. Each superframe contains a frame bit section composed of 12 frame bits, and a payload section composed of 12 samples for each of the 24 DS0 channels. A T1 extended superframe (ESF) is a group of 24 T1 frames. Each ESF frame is composed of an ESF frame bit section that contains 24 frame bits, and an ESF payload section that contains 24 samples of each of the 24 DS0 channels.
Although T1 was developed for voice communications, it is not limited to voice communications. The physical line can carry digitized voice samples, digital computer data, or any other type of data in any combination in the 24 channels. Thus, a broader definition of a T1 link is a digital transmission link with a capacity of 1.544 Mbps.
Information concerning whether a channel is active, idle, ringing, etc., may be passed through the voice channel by borrowing, or robbing, one bit every 6.sup.th frame. This process is called robbed bit signaling. Robbed bit signaling does not noticeably affect the quality of voice connections in a telephone network.
When robbed bit signaling is used and a voice channel is used to carry digital data, only 7 of the 8 bits in each frame can be counted upon to pass data through the network from one end to the other, as the 8.sup.th bit is frequently modified as the robbed bit. This gives rise to a single DS0 channel carrying only 56 Kbps of data. An entire T1 link carrying digital data would carry 1.340 Mbps using this method. This is inefficient and for this reason a new standard for T1 transmission called Primary Rate ISDN (PRI) was developed to more efficiently move digital data through a T1 link.
The PRI format calls for a T1 link to not have robbed bit signaling. Instead, one of the 24 voice channels is dedicated for channel management (e.g. active, ringing, etc.) and is called the "D" channel. The other 23 channels, called bearer channels or "B" channels, may now use all 64 Kbps to carry digital data. An entire T1 link using PRI format can carry 1.430 Mbps. Industry standards frequently call this form of PRI 23B+D. Further efficiencies can arise when multiple T1 links between two end nodes exist. (An end node is a node where a call is originated or terminated; all other nodes along the entire link are used to only route the traffic through the telephone network). A single D channel in one link can carry all the necessary information for several T1 links. Two T1 links with a single D channel would be called 47B+D, and four T1 links would be called 95B+D. Some versions also carry a spare D channel in case the T1 link with the active D channel goes down.
The D channel carries High level Data Link Control (HDLC) messages about the B channels in all the T1 link(s) covered by that particular D channel. When a D channel carries an HDLC message it becomes known as the HDLC channel. Typically, the HDLC channel is the 24.sup.th channel on a T1 link and occupies the 24.sup.th timeslot in a T1 frame. The HDLC channel is used by the DTE equipment at the two end nodes to transmit link management messages. Examples of these link management messages are call setup and call tear-down.
Since T1 is a synchronous TDM link, once a channel connection has been setup between two users, that channel is dedicated until the connection is torn down. This channel dedication is an inefficient use of the 1.544 Mbps of T1 link capacity. For example, assume channel #5 of the 24 T1 channels is set up between user A and user B. Channel #5 will carry all communication between user A and user B. If there is a pause in the communication between user A and user B (such as user A putting user B on hold) during the transmission of a particular T1 frame, then that particular T1 frame will carry an empty channel #5 timeslot. Even a short pause of one minute can lead to 480,000 T1 frames being transmitted with an empty channel #5 timeslot. This is so even if channel #6 is being fully utilized by computer data at 64 Kbps. Because channel #5 is dedicated, the channel #6 user cannot send data over two channels (e.g. #5 and #6) for an effective rate of 128 Kbps.
Asynchronous Transfer Mode (ATM) is an asynchronous type of communications protocol. It is designed to be carried over the emerging fiber optical network, called the Synchronous Optical NETwork (SONET), although it can be carried over almost any communications link. The basic unit of ATM is the ATM cell. Each cell contains two parts; a header, which contains routing information, and a payload, which contains the data to be transported from one end node to another.
ATM is considered asynchronous because each node in the network does not know until after a cell arrives where it is intended to go. In a synchronous network, each timeslot is assigned a certain time when it is to arrive at each node. When it arrives will determine where a timeslot goes. Thus, the individual timeslots do not need to have routing information within them. The arrival of a particular ATM cell at a node, on the other hand, is not guaranteed to occur at a particular point in time.
There are a number of factors which makes ATM attractive to the telecommunications industry. One is the cost of the SONET transport mechanism. On a bit per bit basis, it is significantly less expensive than using metallic links by several factors often. The theoretical capacity of fiber is in excess of 20 tera bits per second (20 million million bits per second). Current technology is at 40 thousand million bits per second, and will soon increase to 160 thousand million bits per second. As technology improves, more information can be sent over each fiber optic buried in the ground.
On the other hand, metallic links that can span long distances and are reasonable to manufacture, have long ago reached their theoretical limits of roughly under 500 million bits per second, and are much bulkier than fiber optic links. The metallic link is also susceptible to rust and corrosion, whereas the fiber is relatively chemically inert. Because of signal attenuation (loss of signal strength as a signal travels down a link) on either type of link, repeaters which re-amplify the signal are needed. Metallic links attenuate the signals more than do fiber links, so more repeaters for metallic links are needed than for fiber links for a given distance. For instance, a T1 link can span a maximum of just over one mile (6000 feet) before a repeater is needed. It is not unusual for fiber optic links to span 50 to 100 miles between repeaters.
For this reason, it is now cost effective for two end nodes of a T1 link to convert their T1 signals into ATM cells, transport it across a SONET network, and then reconvert back to T1 at the other end. At the destination node, an ATM receiver unloads the T1 frames from the ATM cells. The ATM receiver sends the T1 frames to a switch matrix where the DS0 channels are de-multiplexed and sent to their particular user destinations. This approach is referred to as T1 emulation over an ATM network (or T1 over ATM, for short).
Conventional methods of T1 emulation over ATM involve transporting the T1 frame bit and T1 payload of particular T1 frame in the payload of an ATM cell. When a number of ATM cells are used to carry a stream of T1 frames, the T1 frame bit position(s) in the ATM cell payload vary over consecutive ATM cells. This occurs because a T1 frame is 193 bits in size and the payload of an ATM cell is 48 bytes or 384 bits in size. Thus, the payload of an ATM cell carries one complete T1 frame and a fraction of another T1 frame causing the T1 frame bit position(s) to vary from ATM cell to ATM cell.
In conventional T1 emulation, the T1 frame bit variation requires that the T1 frames be synchronized after they are unloaded from the ATM cells at the destination node. Synchronization is necessary to determine the location of the T1 frame bits, which serve to separate the T1 frames. Otherwise, the destination node switch matrix cannot de-multiplex the T1 frames because it is not known where one T1 frame ends and another begins.
In conventional T1 emulation, a T1 framer synchronizes a quantity T1 frames in ESF format by aligning the T1 data bit stream in 772 columns. This process is know as T1 framing. The T1 framer searches each column for a specific frame bit pattern that identifies the a column of frame bits. Once this pattern is found, the T1 frame bit positions are identified and the following T1 frames are synchronized.
The significance of the 772 columns is that one synchronization bit occurs every 772 bits of a 4632 bit ESF frame. Thus, an entire ESF frame occupies 6 rows of 772 columns. When properly synchronized, one of these columns will contain all 6 synchronization bits.
When the T1 frames are in ESF format, a second layer of synchronization is required to find the T1 frame #1 of the 24 T1 frames in an ESF frame. This process is known as ESF framing and is done by an ESF framer, as opposed to a T1 framer. In other words, the T1 framer determines the location of the T1 frame bits so that successive T1 frames can be distinguished, and the ESF framer determines T1 frame #1 in an ESF frame so that successive ESF frames can be distinguished.
Both T1 framing and ESF framing must be done whenever synchronization is lost for whatever reason. However, T1 framing requires more complicated hardware and takes longer than ESF framing because a greater number of bits must be examined. What is needed is a method and system of T1 emulation over ATM that reduces the amount of framing required when synchronization is lost.